1. Field of the Invention
The present invention relates to an electromagnetic relay having a permanent magnet to exert a force on an armature opposite a coil.
2. Description of the Prior Art
In German Pat. No. OS 1 439 210 is disclosed a relay having an excitation coil about a core with the yoke fastened to one end of the core and an armature forming a working air gap at the other end of the core. A permanent magnet is provided to exert a restoring force on the armature, at least by one pole of the magnet. Thus, a neutral, monostable magnet system is formed wherein the armature is attracted in opposition to the restoring force of the permanent magnet for the chronological duration of the coil excitation, and is returned to its quiescent position by the attraction of the permanent magnet upon discontinuation of the coil excitation.
The relay disclosed preferably includes a ceramic magnet as a restoring means, such ceramic magnets are also referred to as oxide magnets or ferrite magnets. Such magnets exhibit a relatively great temperature response. In particular, they have a temperature coefficient of remanence on the order of magnitude of -2.times.10.sup.-3 .degree.K.sup.-1. This means that the permanent magnetic flux decreases with increasing temperature, the flux decreasing by about 10% for a temperature rise of 50.degree. K. The negative temperature coefficient of the remanence of a permanent magnet is used in polarized, bistable relays to compensate the positive temperature coefficient of the coil resistance so that the response voltage increases less greatly for increasing temperatures than in an unpolarized relay. See, for example, Siemens Zeitschrift, 1969, pages 411-413.
In polarized relays, it is known to compensate a part of the permanent magnetic force by using springs to produce an optimum approximation of the temperature response of the permanent magnet to the temperature response of the coil. See, for example, German Pat. No. AS 1 902 610.
Since such relays are generally excited only by pulses, there is no significant intrinsic heating of the coil, and the temperature of the coil and the permanent magnet essentially correspond to the ambient temperature so that any temperature compensation of such bistable polarized relays is with respect to ambient temperature.
For a neutral magnetic system of the type described above, the monostable relay exhibits a high coil heating for longer duration excitations. Since the permanent magnet which provides an armature restoring force is at a distance from the heated coil and is largely at ambient temperature, the temperature compensation in the known polarized relays is not identically provided for such conditions.